docs: fixed links to tokio_timer::clock::Now (#842)
docs: fixed links to tokio_timer::clock::Now (#842)

* docs: fixed links to tokio_timer::clock::Now in tokio-timer/src/timer/mod.rs

* docs: fixed links to std::time::Instant in tokio-timer/src/timer/mod.rs

tokio-rs/tokio,tokio-rs/tokio